Chytridiomycetes are a class of fungi. Members can be found in the ground, freshwater, and salt estuaries. They are first known from the Rhynie chert. It has recently been redefined to exclude the taxa Neocallimastigomycota and Monoblepharidomycetes, which are now a phylum and a sister-class respectively.
Chytridiomycetes is the major class of the phylum Chytridiomycota. The member of the class Chytridiomycetes, commonly called chytrids.
What are The Habitat of Chytridiomycetes?
Chytrids have been isolated from a variety of aquatic habitats including peat, bogs, rivers, ponds, springs, and ditches, and terrestrial habitats such as acidic soils, alkaline soils, temperate forest soils, rainforest soils, arctic and Antarctic soils.
The fungi included in this class are usually water-inhabiting fungi. As parasites or saprobic generally found in water and also in soil. As parasites, they grow internally in algae, microfauna, and water molds. These chytrids often cause the destruction of unicellular water algae.
They also parasitize the terrestrial higher plant (synchytrium endobioticum on a potato). A few chytrids parasitize animal eggs and protozoa, while others are saprobic on the decaying remains of plants.
Classification of Chytridiomycetes.
Chytridiomycota, a phylum of fungi (kingdom Fungi) characterized by zoospores (mobile cells) with a single posterior whiplash structure (flagellum). The species are microscopic in size, and most are found in freshwater or moist soils. Most are parasites of algae and animals or live on organic debris (as saprobes).
The analyses of Multigene phylogenetic analyses, new culture techniques, and additional collections of Chytridiomycetes have revealed greater diversity and led to increasing the number of orders in which to classify about 700 species under 90 genera.
Today there are 10 described orders of Chytridiomycetes:
We will discuss each order in a separate article.
Phylogenetic classification of Chytridiomycetes.
What is Life Cycle of Chytridiomycetes?
The Chytridiomycota, often called chytrids, are unique among all fungi in having motile stages in their life cycles; no other fungi have this trait. These motile stages take the form of zoospores, single cells with a single posterior (at the rear) flagellum.
As we discuss above members of class Chytridiomycetes are parasitic which means it leaves on or in the host cell. We discuss the life cycle of chytrids with an example of the well-studied species Chytriomyces hyalinus. This fungus forms a well-developed rhizoidal system within its substrate.
The sporangium that develops from the encysted zoospore has a saucer-shaped operculum from which zoospores escape into a fibrous vesicle of overlapping filaments where the cells complete their maturation and then escape.
The escaped zoospores encyst and germinate to form new sporangia and rhizoids (asexual thalli) or to function s sexual thalli. This is one of the few members of chytridiomycetes in which sexual reproduction is well documented.
In the sexual reproduction of chytrids, the rhizoids of two thalli come into contact and fuse, and the resting spore forms the junction of the rhizoidal anastomosis. The resulting sporangium develops a thick wall and eventually germinates by the production of zoospores, apparently after meiosis.
What is Characteristics of Chytridiomycetes?
1) Flagellated spore
The main characteristic of this class is the production of uniflagellated reproductive cells known as zoospores and planogametes. The single flagellum is of the whiplash-type and is inserted posteriorly. The zoospore with a flagellum inserted posteriorly is called an opisthokont.
The flagellum is attached to the blepharoplast in the cell. The motile cells of some species possess a nuclear cap which consists of RNA. It shielded the nucleus at the anterior end of the cell.
2) Cell wall and Rhizoids
It has no cell wall in the earlier stages (Olpidium). In the more advanced species, the unicellular thallus is pulled into fine, branched hair at one point, the so-called rhizoids, which support the anchoring and absorption of nutrients (rhizophidium).
In slightly more complex members, there is a much-branched rhizomycelium. The scanty mycelium consisting of a few short filamentous hyphae is in evidence.
Some members of this class have a mycelium consisting of typical hyphae woven into a eucarpic mycelial network (Monoblepharis). The hyphae are coenocytic. Chitin is a main component of the hyphal cell wall. Besides chitin, there is β glucan.
3) Mode of Reproduction
As we discussed above, The asexual reproductive organ is the sporangia, each of which produces numerous tiny, uninucleate, and uniflagellate opisthocont zoospores.
The liberated zoospores swim for a time. Later each retracts its flagellum and undergoes encystment. After a short period of rest, the encysted spore germinates. Sexual reproduction may be isogamous or anisogamous. In some, it is typically oogamous.
Disease caused by Chytridiomycetes
The chytrid Batrachochytrium dendrobatidis is responsible for chytridiomycosis, a disease of amphibians. This disease was discovered in Australia and Panama in 1998 and is known to kill amphibians in large numbers. It was thought to be the main cause of the global decline in amphibians.
It is estimated that 30% of the world’s amphibian species will experience severe population decline or extinction.
I) Most of the Soil inhabiting Chytridiomycetes attack the underground as well as aerial part of the higher plant and cause diseases that are of great economic significance.
For example, Synchytrium endobioticum causes black wart disease of Potato, Urophlyctis alfalfa crown wort of alfalfa (Medicago), and Physoderma maydis causes brown spot disease of maize (Zea mays).
II) Many chytrids indirectly harm humans and animals. They parasitize and destroy the phytoplanktonic forms of algae that form an important link in the food chain of aquatic ecosystems.